Conversion of biomass to value-added products, especially to different forms of energy has received growing attention as a mean of replacing energy and other end-products derived from fossil raw materials. For instance, of the conventional biofuels for transport (ethanol, ETBE, pure vegetable oil, biodiesels and biomethane), ethanol has a long proven history and in some cases also environmental advantages compared to fossil fuels. In addition, biomass is considered a major renewable source for bulk chemicals and materials.
Biomass can also be converted into various chemicals by fractionating processes yielding its components either in native or modified (e.g. hydrolysed or oxidised) form. The mixture of components obtained by such a conversion can be used as chemicals as such or after further processing. The further processing methods can be physical or mechanical (e.g. fractionation and separation), chemical (e.g. use of components as raw materials for synthesis or chemical modifications) or biochemical (e.g. fermentation processes or enzymatic modification). It is thus possible to utilise all the most abundant components of biomass: cellulose (or glucose derived from it), hemicellulose (or sugars or oligosaccharides derived from it), lignin (or its fragments) and minor biomass components depending on the specific raw materials used (e.g. proteins. pectins, extractives), and their degradation products.
Conventional methods of fractionation of biomass in order to produce sugars by enzyme hydrolysis or products based on further processing of sugars require a pretreatment to be carried out e.g. to render the biomass amenable to hydrolysis.
International Patent Application WO 1994/03646 describes a method of pretreating lignin-containing biomass to render the biomass amenable to digestion. In the method water and calcium hydroxide and optionally oxygen containing gas are added to the biomass at a temperature of 40 to 150° C. and an elevated pressure at a pH between 8.5 and 10.5. The aim of the known method is achieve oxidation without degradation of the lignocellulose.
WO 2004/081185 describes methods of hydrolysing lignocellulose at mild or moderate conditions, such as at a temperature of about 10 to 90° C. and a pH of about 4.0 to 10.0. A treatment using e.g. sodium percarbonate or Na2CO3 in a pre-treatment step is shown to increase sugar yield when compared to sole enzymatic treatment. A wide range of oxidizing agents, including hydrogen peroxide, sodium and calcium hypochlorite and potassium permanganate, are used in the examples.
An improved pretreatment method is disclosed in International Patent Application No. WO 2011/061400, which describes a process wherein lignocellulosic material and an alkaline agent are treated with an oxidising agent in water to produce a solid fraction with reduced lignin content and a liquid fraction comprising lignin.
Oxidation of chips at alkaline conditions as a pretreatment method is discussed by Rovio, S. et al. and Kallioinen A, et al. Further art is disclosed in US 2002/148575, CN 102603504, US 2008/032344, Sun et al. and Alatriste-Mondragon, F. et al.
The main difficulties associated with existing techniques are incomplete decomposition or modification of the lignocellulosic material and the formation of toxic compounds during pre-treatment, which makes the further processing less feasible and inefficient. In particular, the crude hemicellulose or cellulose filtrate from conventional pretreatments usually contains various degradation products of lignocellulose. These can be lignin and sugar decomposition products, including furfural, hydroxymethyl furfural and formic and acetic acid. Most of these components are toxic to enzymes and microorganisms and will slow any subsequent hydrolysis and fermentation process. Ineffective enzymatic hydrolysis step reduces the process economy remarkably.